@Auld_fella Welcome to the Forum.
I too had dodgy knees before buying my first emtb. I was down to 10 miles on fairly flat terrain on my mtb and then needed 3 days to recover! With my emtb I can now easily do 2-3 times that and do it every day. The bonus is that because I am no longer stressing my knees quite as much, it feels as though they are improving!!
PS: I can recommend being a bit more specific about what part of the UK you are based as it may lead to invitations to ride from others in your area.